Donated art works of former South African president Nelson Mandela are pictured at the newly renovated Nelson Mandela Centre of Memory in Houghton, Johannesburg, June 11, 2013. A number of documents, including Mandela's first official passport and letters he wrote to his family and friends while he was a prisoner at Robben Island, are kept at the centre.(SIPHIWE SIBEKO/REUTERS)

Awards given to former South African president and anti-apartheid leader Nelson Mandela are pictured at the newly renovated Nelson Mandela Centre of Memory in Houghton, Johannesburg, June 11, 2013.(SIPHIWE SIBEKO/REUTERS)

Books are pictured in former South African president and anti-apartheid leader Nelson Mandela's office at the newly renovated Nelson Mandela Centre of Memory in Houghton, Johannesburg, June 11, 2013. Mandela's autobiography Long Walk to Freedom is among the books.(SIPHIWE SIBEKO/REUTERS)

Framed pictures hang in the office of former South African president and anti-apartheid leader Nelson Mandela at the newly renovated Nelson Mandela Centre of Memory in Houghton, Johannesburg, June 11, 2013.The picture at left by photographer Peter Magubane shows Mandela (hand raised) with late African National Congress (ANC) president Oliver Tambo, while at right is a portrait of Mandela by photographer Peter Morey.(SIPHIWE SIBEKO/REUTERS)

Calendar entries recording blood pressure levels (BP) of former South African president and anti-apartheid leader Nelson Mandela while he was in jail is seen at the newly renovated Nelson Mandela Centre of Memory in Houghton, Johannesburg, June 11, 2013.(SIPHIWE SIBEKO/REUTERS)

A general view shows the desk of former South African president and anti-apartheid leader Nelson Mandela at the newly renovated Nelson Mandela Centre of Memory in Houghton, Johannesburg, June 11, 2013.(SIPHIWE SIBEKO/REUTERS)

A photograph of U.S. President Barack Obama meeting former South African president Nelson Mandela is seen at Mandela's office at the newly renovated Nelson Mandela Centre of Memory in Houghton, Johannesburg, June 11, 2013.(SIPHIWE SIBEKO/REUTERS)

A miniature of former South African president Nelson Mandela as a boxer is pictured below framed pictures of his late sons, Makgatho (L and alone in right frame) and Madiba Thembekile Mandela (Thembi) by photographer Eli Weinberg, in Mandela's office at the newly renovated Nelson Mandela Centre of Memory in Houghton, Johannesburg, June 11, 2013.(SIPHIWE SIBEKO/REUTERS)

A boxing glove donated and signed by Laila Ali, daughter of retired heavyweight boxing legend Muhammad Ali, is seen at the office of former South African president and anti-apartheid leader Nelson Mandela at the newly renovated Nelson Mandela Centre of Memory in Houghton, Johannesburg, June 11, 2013.(SIPHIWE SIBEKO/REUTERS)

The unfinished works of former South African president and anti-apartheid leader Nelson Mandela, written after his auto biography Long Walk to Freedom, is seen at the newly renovated Nelson Mandela Centre of Memory in Houghton, Johannesburg, June 11, 2013.(SIPHIWE SIBEKO/REUTERS)

The first official passport of former South African president and anti-apartheid leader Nelson Mandela is seen at the newly renovated Nelson Mandela Centre of Memory in Houghton, Johannesburg, June 11, 2013.(SIPHIWE SIBEKO/REUTERS)

School children walk past a sign with the image of former South African President Nelson Mandela in Qunu, June 11, 2013.(ROGAN WARD/REUTERS)